1. Field of the Invention
The system of this invention relates to the demodulation of piecewise coherent, combined frequency and phase-shift-keyed signals. More particularly, this system applies to the demodulation of such signals that are modulated by orthogonal or bi-orthogonal codes. Examples of orthogonal codes used to modulate such signals are described in "Codes with Special Correlation" by Leonard Baumert which appears as Chapter 4 in "Digital Communication with Space Applications" edited by Soloman Golomb, Prentice-Hall, Englewood Cliffs, N.J. (1964).
2. Description of the Prior Art
Prior art systems exist for the demodulation of coherent, phase-shift-keyed signals. However, in some applications, such as in "frequency-hopping" communication systems, the received signals do not exhibit phase coherence over long intervals. In frequency-hopping systems, the transmission frequency is changed abruptly a number of times during transmission. Typically, the changes in frequency are a great many times greater than that normally associated with frequency shift keying. Because of the large changes in frequency, and the variations of propagation conditions with frequency, phase-coherence between the different transmission frequencies, typically does not obtain. However, the received signal normally still exhibits phase-coherence during the interval of transmission on a single frequency. The period of transmission at a single frequency, normally is sufficiently long to include the transmission of one, and typically a number, of code words. U.S. patent application Ser. No. 4,485, filed Jan. 8, 1979, for the invention "IN-PHASE QUADRATURE DEMODULATOR FOR CPSK SIGNALS" describes a system for demodulating such frequency-hopped signals where phase coherence is maintained only for the short intervals containing one or more code words of transmitted data. A system also has been developed for the modulation and demodulation of signals by a combination of frequency and phase-shift-keying. Such a combined system is described in U.S. patent application Ser. No. 880,631, filed Feb. 23, 1978, now abandoned and replaced by continuation-in-part Ser. No. 50,795, filed June 21, 1979. The system of the invention described in the present application is an adaptation of the "in-phase quadrature" demodulation technique for use in the demodulation of combined, frequency and phase-shift-keyed signals which exhibit piecewise phase coherence over the intervals occupied by one or more code words, but which may lack phase coherence between the portions of the signal received at different frequencies because of frequency hopping.